A number of systems have been proposed for electricity storage that store the heat of compression of air and absorb the work of expansion of air.
A commonly proposed example of this is called Adiabatic CAES where a salt cavern is typically used as a compressed air store. When electricity is to be stored a motor drives a compressor to compress air into the cavern. The compression process raises the temperature of the air and to allow efficient energy recovery it is necessary to store this ‘heat of compression’ in some form of thermal store.
The cavern will normally be kept at a minimum pressure, such as 40 bar, and this is increased to a higher limit, for example 60 bar, during charging. These pressures are likely to generate a peak temperature, using air, in the region of 650 degrees C. This is normally either transferred to an unpressured thermal store by a heat exchanger or stored directly in a thermal storage matrix contained within a pressurised vessel. To recover the electricity the process is reversed and the compressed gas is reheated by the thermal store prior to expansion. The work of expansion is used to drive a generator to generate electricity.
The aim is to store the heat with only a small difference between the compressed air temperature and the storage material temperature, such that when the process is reversed the air is heated to near its original temperature.
As mentioned, one option is to use a heat exchanger rather than a thermal storage matrix within a pressurised vessel. However, this sort of heat exchange is extremely difficult to achieve because there are no heat transfer liquids that operate in the range 0-650 degrees C. This means that either multiple liquids must be used or the heat exchange is via a gas, which means a gas to gas heat exchanger.
Multiple heat transfer liquids are difficult to manage, require multiple storage vessels and are generally expensive, but they can operate efficiently and avoid the cost of heavily pressurised vessels.
With gas to gas heat exchangers the temperature range requires the use of quality steels and the gas flows require very large heat exchangers to avoid pressure drop. The result of this is that these heat exchangers are normally both very expensive and not very efficient, with a large temperature difference, such as 50 degrees C., after each heat transfer process.
The most efficient solution is to use a thermal storage matrix, such as a particulate structure, contained within an insulated pressure vessel and to transfer the heat to and from the gas in a manner that is similar to a very large regenerator. This has the best heat transfer, but the storage mass must all be contained within the pressure vessel, which is very expensive.
Heat transfer within a packed bed or porous media is normally a function of surface area. The higher the surface area the better the heat exchange. If smaller particles or channels or pores are used then the surface area tends to increase per unit volume of storage material—it is said to have a higher ‘specific surface’. For example:                Packed bed spheres 10 mm diameter (cubic packing) approx 314 m2/m3         Packed bed spheres 1 mm diameter (cubic packing) approx 3140 m2/m3         Porous metal foam 5 pores per inch (12% density) approx 430 m2/m3         Porous metal foam 40 pores per inch (12% density) approx 2100 m2/m3 This shows that packed spheres with 1 mm particle size have a specific surface of approximately 3140 m2 or surface area in each cubic meter. For the porous foam metal with 40 pores per inch there is a specific surface of 2100 m2 of surface area in each cubic meter. The density of the foam metal is 12% of the solid, which means that it has a void fraction of 88%. The void fraction of the spheres in this example is approximately only 50% by way of comparison.        
There is a further advantage of a higher specific surface. Without a temperature difference between two objects there can be no heat exchange. This temperature difference must lead to irreversible thermal mixing which has no impact on the total quantity of heat stored, but does reduce the temperature at which it is stored. This in turn reduces the amount of energy that can be recovered from the stored heat as the quality of the heat has been degraded. This degradation should be distinguished from a simple loss of heat to the environment through the insulated walls of the store.
This degradation is created because there must be a temperature difference between the gas and the particle in the store, so the particles are always slightly cooler than the gas when being charged (in a hot store). When the system is discharged and the gas is blown back in the reverse direction the gas must now be cooler than the particle and hence the gas comes back out of the thermal store at a lower temperature, if it is a hot storage vessel and at a higher temperature if it is a cold storage vessel. This degradation can be regarded as the result of certain irreversible processes and these have a loss associated with them, which in an energy storage scheme results in a reduction of the amount of electricity recovered. These ‘irreversible’ thermal losses can be reduced by reducing the particle size, but this increases gas pressure losses through the stores.
In a heat storage situation, a ‘thermal front’ is created in the storage vessel, i.e. a rise or a fall in temperature in the storage media and/or the gas with distance moved downstream, which occurs in the region of the store where thermal transfer is most active. FIG. 10 illustrates the formation of a thermal front in a thermal store and shows how the process of charging a thermal store sets up a thermal front within a region of the store that progresses downstream and that is usually initially quite steep but which becomes progressively shallower as charging continues. Thus, the front starts with length L1, but as it moves down the vessel it extends in length to length L2 and then L3. As the front will usually be asymptotic, the length of the front can be discussed in terms of the length of the front between TH2 and TA2, these being within 3% of the peak temperature and start temperature. If different criteria are set i.e. within 2% of the peak and start temperatures, then the nominated front lengths will be slightly longer.
For a certain store geometry a longer front will give lower thermal losses, but the length of the front will also reduce the useable amount of the store i.e. it will reduce the store utilization. If a store is 5 m in diameter and 10 m long and the thermal front is 5 m of this length, then the store utilization is reduced to approximately 50%.
If the same sized store was used and the particle size was reduced, then the same level of thermal losses could be achieved with a much shorter front. So a smaller particle size in a packed bed or pore size in a porous media will tend to give better heat transfer, lower thermal losses and better store utilization (a shorter thermal front). The one disadvantage is that there is a pressure drop associated with the fluid flow through the bed and this pressure drop increases significantly as the particle or pore size reduces.
The resistance to fluid flow increases with a decrease in the particle size and gives rise to a pressure drop in the fluid (dP). Pressure is not a vector quantity, but a pressure gradient may be defined with respect to distance. In the case of a thermal store there is a certain pressure drop dP over a store of length L, which in this case means the pressure gradient is dP/L. The pressure decreases in the direction of the fluid velocity so the gas pressure will be lower after the gas has passed through the store. This pressure drop is also the reason why the particle size in packed beds is not reduced to a very small size that will give much higher thermal reversibility. The losses from the pressure drop outweigh the benefits of the smaller particle size.
Accordingly, the present applicant has appreciated the need for an improved thermal storage system which overcomes, or at least alleviates, some of the problems associated with the prior art.